Epoxy resin compositions each containing an epoxy compound and a curing agent for the epoxy compound are widely used in laminated-plate resin materials, electric insulating materials, semiconductor encapsulating materials, fiber-reinforced composite materials, coating materials, molding materials, adhesive materials, etc. in view of the excellent physical properties such as heat resistance, moisture absorption resistance, and the like.
In recent years, in these various applications, particularly in the field of advanced materials, performance characterized by heat resistance, moisture absorption resistance, and low coefficient of thermal expansion property has been required to be further improved. Further, according to regulations to environmental problems and the like, high-melting-point solders not using lead (lead-free solders) have become mainstream, and the working temperatures of the lead-free solders are about 20° C. to 40° C. higher than usual eutectic solders, thereby requiring epoxy resin cured products to have higher heat resistance and higher moisture absorption resistance than ever.
Examples of known epoxy resin materials which can comply with the requirements for high degrees of heat resistance and moisture absorption resistance and low coefficient of thermal expansion property include tetrafunctional naphthalene-based epoxy compounds represented by a structural formula below (Patent Literature 1).

In comparison with general phenol novolac-type epoxy compounds, the tetrafunctional naphthalene-based epoxy compounds have a naphthalene skeleton having high heat resistance and high hydrophobicity, a high crosslink density due to tetrafunctionality, and a molecular structure with excellent symmetry, and thus cured products thereof exhibit very excellent heat resistance, moisture absorption resistance, and low coefficient of thermal expansion property. However, higher performance has recently been required for resistance, and further improvement has been required. Further, since the tetrafunctional naphthalene-based epoxy compounds have low solubility in solvents, the characteristics of the cured products have been not sufficiently exhibited in, for example, manufacture of printed circuit boards.
It is considered to be effective as a method for improving heat resistance that in the tetrafunctional naphthalene-based epoxy compounds, naphthalene rings are directly bonded to each other, not bonded through a methylene structure. There is described an epoxy compound having a bi(dihydroxynaphthalene) structure in which a dihydroxynaphthalene dimer has a direct single bond without containing a methylene structure (Patent Literatures 2 to 5). The positions of hydroxyl groups in dihydroxynaphthalene and the bond position in a dimer are important factors which influence the physical properties of an epoxy resin using the compound, such as the softening point, solvent solubility, the heat resistance of cured products thereof, and the like. However, any one of Patent Literatures 2 to 5 does not specify the positions of hydroxyl groups in dihydroxynaphthalene and the bond position in a dimer, and does not specifically describe the compound.
Synthesis of [1,1′-binaphthalene]-2,2′,7,7′-tetraol generally uses coupling reaction of dihydroxynaphthalene or a dihydroxynaphthalene derivative. This reaction is selective coupling reaction at the 1,1′-positions of 2,7-dihydroxynaphthalene, and thus [1,1′-binaphthalene]-2,2′,7,7′-tetraol with high purity can be produced, thereby producing an epoxy compound exhibiting excellent performance such as a low softening point, low melt viscosity, high solvent solubility, and the like. Also, an epoxy compound produced from [1,1′-binaphthalene]-2,2′,7,7′-tetraol of the present invention was not synthesized in the past and is thus a novel epoxy compound.
[1,1′-Binaphthalene]-2,2′,7,7′-tetraol has a molecular structure very similar to that of 1,1-bis(2,7-dihydroxy-1-naphthyl)alkane described in Patent Literature 6, but has excellent heat resistance and handleability at a low temperature because it does not have an alkylene group, which is unstable under high-temperature conditions, and has a lower melting point than 1,1-bis(2,7-dihydroxy-1-naphthyl)alkane. Also, [1,1′-binaphthalene]-2,2′,7,7′-tetraol can take the forms of an anhydride and hydrates such as monohydrate and dehydrate. The anhydride has a melting point of 218° C., while the monohydrate has a softening point of 124° C. as well as a melting point and thus application in a lower temperature region can be expanded.
Non-Patent Literatures 1 to 3 describe examples of synthesis of [1,1′-binaphthalene]-2,2′,7,7′-tetraol. The products of oxidative coupling reaction of naphthalene-2,7-diol include [1,1′-binaphthalene]-2,2′,7,7′-tetraol, unreacted raw materials, and multimers such as a trimer, a tetramer, and the like, which have similar dissolution behaviors in a solvent due to similar structures and thus cannot be easily separated from each other. Therefore, recrystallization with carbon disulfide, benzene, and ethanol described in the literatures produces mixed crystals of [1,1′-binaphthalene]-2,2′,7,7′-tetraol and impurities, and [1,1′-binaphthalene]-2,2′,7,7′-tetraol with high purity cannot be produced. This is a cause for failing to obtain expected values of physical properties, such as high heat resistance and moisture resistance, of resins obtained by using the compound.